Two-Steps Method to Prepare Multilayer Sandwich Structure Carbon Fiber Composite with Thermal and Electrical Anisotropy and Electromagnetic Interference Shielding

Carbon fiber (CF) composites performance enhancement is a research hotspot at present. In this work, first, a sandwich structure composite, CF@(carbon nanotube/Fe3O4)/epoxy (CF@(CNT/Fe3O4)/EP), is prepared by the free arc dispersion-CFs surface spraying-rolling process method, herein, CFs in the middle layer and (CNT/Fe3O4)/EP as top and substrate layer. Then, CF@(CNT/Fe3O4)/EP (on both sides) and CFs (in the middle) are overlapped by structure design, forming a multilayer CF@(CNT/Fe3O4)/EP-CFs composite with a CFs core sheath. A small amount of CNT/Fe3O4 is consumed, (CNT/Fe3O4)/EP and CFs core sheath realize thermal and electrical anisotropy and directional enhancement, and multilayer sandwich structure makes the electromagnetic interference (EMI) shielding performance better strengthened by multiple absorption–reflection/penetration–reabsorption. From CF-0 to CF-8, CNT/Fe3O4 content only increases by 0.045 wt%, axial thermal conductivity (λ‖) increases from 0.59 W/(m·K) to 1.1 W/(m·K), growth rate is 86%, radial thermal conductivity (λ⊥) only increases by 0.05 W/(m·K), the maximum λ‖/λ⊥ is 2.9, axial electrical conductivity (σ‖) increases from 6.2 S/cm to 7.7 S/cm, growth rate is 24%, radial electrical conductivity (σ⊥) only increases by 0.7 × 10−4 S/cm, the total EMI shielding effectiveness (EMI SET) increases by 196%, from 10.3 dB to 30.5 dB. This provides a new idea for enhancing CFs composite properties.


Introduction
With the rapid development of aerospace, transportation, energy, medical and health fields, there is an urgent need for materials with excellent thermal/electrical conductivity properties and electromagnetic interference shielding effectiveness (EMI SE) to adapt to the work in complex environments. Carbon fiber (CF) composites are widely used because of its high strength, high modulus, light weight and easy molding [1][2][3][4][5]. However, the poor magnetic property for CFs limits the further improvement of EMI SE [6]. In addition, the epoxy resin (EP), which is often used as the matrix of CF composites, has advantages of light weight, designability and easy processing [7], but its low intrinsic thermal conductivity (0.1~0.4 W/(M·K)) [8,9] and EMI SE (about 2 dB) [10] limit the performance of CF composites. Therefore, the preparation of CF composites with excellent thermal/electrical conductivity properties and EMI SE has become a research hotspot.
Adding nanofillers is one of the effective methods to prepare high performance composites [11][12][13]. Carbon nanotubes (CNTs) have excellent thermal and electrical properties [14,15] and are often used as an ideal material to enhance thermal/electrical conductivity properties of CF composites [16,17]. Moreover, because of their good dielectric loss

Experiments
Step1, free arc dispersion-CFs surface spraying-rolling process method CNTs, Fe 3 O 4 and DI water were mixed at a mass ratio of 1:3:10 and thoroughly stirred for 10 min, putting in the mold and applying 10 kg pressure to extrude into a cylindrical block (diameter 30 mm and height 10 mm). According to the free arc dispersion method of Li et al. [35,36], the cylindrical block was placed between the high-voltage pulse electrodes for dispersion, the voltage was 12 KV, the frequency was 10 Hz, the positive electrode used titanium grid, the negative electrode used titanium plate and CNT/Fe 3 O 4 dispersion fog was obtained. At the same time, CNT/Fe 3 O 4 dispersion fog passed through the spraying channel and was sprayed on continuously moving CFs surface by negative pressure airflow traction, and CFs movement speed was 0.01 m/s. The sprayed CFs moved into the heating box for heating at 100 • C to obtain CF@(CNT/Fe 3 O 4 ). Finally, EP was poured on CF@(CNT/Fe 3 O 4 ), after rolling, CNT/Fe 3 O 4 was laid on the CFs surface to construct CNT/Fe 3 O 4 network and CF@(CNT/Fe 3 O 4 )/EP was prepared. The above processes were simultaneous and continuous.
Step2, structure design Pure CFs was placed in the middle as the core sheath, and CF@(CNT/Fe 3 O 4 )/EP was overlapped on the upper and lower sides of pure CFs, putting into the mold, and transferring to the heating box for curing, multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite was obtained. Curing temperature T 1 = 150 • C, T 2 = 180 • C, curing time T 1 = 2 h, T 2 = 2 h. The sample size was 2 mm × 12 mm × 20 mm.
After calculation, the CFs volume fraction was 60% and the volume fraction of EP was 40% in the sample. Considering the sample performance gradient and consistency of composite size, the total number of layers for CF@(CNT/Fe 3 O 4 )/EP and pure CFs was fixed to 8. The schematic diagram of CFs overlapping method and the description of treatment for each experimental group were shown in Table 1.

Experiments
Step1, free arc dispersion-CFs surface spraying-rolling process method CNTs, Fe3O4 and DI water were mixed at a mass ratio of 1:3:10 and thoroughly stirred for 10 min, putting in the mold and applying 10 kg pressure to extrude into a cylindrical block (diameter 30 mm and height 10 mm). According to the free arc dispersion method of Li et al. [35,36], the cylindrical block was placed between the high-voltage pulse electrodes for dispersion, the voltage was 12 KV, the frequency was 10 Hz, the positive electrode used titanium grid, the negative electrode used titanium plate and CNT/Fe3O4 dispersion fog was obtained. At the same time, CNT/Fe3O4 dispersion fog passed through the spraying channel and was sprayed on continuously moving CFs surface by negative pressure airflow traction, and CFs movement speed was 0.01 m/s. The sprayed CFs moved into the heating box for heating at 100 °C to obtain CF@(CNT/Fe3O4). Finally, EP was poured on CF@(CNT/Fe3O4), after rolling, CNT/Fe3O4 was laid on the CFs surface to construct CNT/Fe3O4 network and CF@(CNT/Fe3O4)/EP was prepared. The above processes were simultaneous and continuous.
Step2, structure design Pure CFs was placed in the middle as the core sheath, and CF@(CNT/Fe3O4)/EP was overlapped on the upper and lower sides of pure CFs, putting into the mold, and transferring to the heating box for curing, multilayer CF@(CNT/Fe3O4)/EP-CFs composite was obtained. Curing temperature T1 = 150 °C, T2 = 180 °C, curing time T1 = 2 h, T2 = 2 h. The sample size was 2 mm × 12 mm × 20 mm.
After calculation, the CFs volume fraction was 60% and the volume fraction of EP was 40% in the sample. Considering the sample performance gradient and consistency of composite size, the total number of layers for CF@(CNT/Fe3O4)/EP and pure CFs was fixed to 8. The schematic diagram of CFs overlapping method and the description of treatment for each experimental group were shown in Table 1.

Characterizations
Field emission scanning electron microscope SEM (SU-8010, Hitachi, Tokyo, Japan) was applied to observe the surface distribution and morphology of CFs and composites. Raman spectrometer (InVia Reflex, Renishaw, London, UK) was used to analyze the material structure of CNT and Fe 3 O 4 , and the laser wavelength was 532 nm. X-ray diffractometer XRD (MiniFlex 600, Rigaku, Tokyo, Japan) was used to characterize the atomic structure of CNT and Fe 3 O 4 , and the scanning speed was 10 • /min, the range was 20-80 • . Thermal constant analyzer (TPS2500S, Hot Disk, Uppsala, Sweden) was used to test the thermal conductivity of multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite according to the standard of ISO22007-2-2015. Electrical conductivity of multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite was measured by four probes resistance tester (RTS-8, Guangzhou Four Probes Technology, Guangzhou, China), and micro-current tester (ST2643, Suzhou Jingge, Suzhou, China) was used to test interlaminar resistivity of multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs compos-ite. Vibrating sample magnetometer VSM (7404, LakeShore, OH, USA) was employed to test the magnetization hysteresis loops of CFs, CNT/Fe 3 O 4 and CF@(CNT/Fe 3 O 4 )/EP at room temperature. Vector network analyzer (ZNB20, Rohde & Schwarz, Munich, Germany) was employed to measure the S 11 , S 22 , S 12 and S 21 parameters of multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite according to the standard of ASTM D5568-08, frequency was X-band . the total EMI SE (SE T ), reflection EMI SE (SE R ) and the absorption EMI SE (SE A ) of multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite were calculated according to the following formula [44]: As is shown in Figure 2a, the left side is pure CFs, and the middle and right side are the CFs sprayed with CNT/Fe 3 O 4 dispersion fog. It can be clearly seen that pure CFs has a light color and luster, but the CFs sprayed with CNT/Fe 3 O 4 dispersion fog appears darker color. This is because the adsorption of CNTs and Fe 3 O 4 on the CFs surface and changes the diffuse reflection of CFs surface. Figure 2b shows that pure CFs has a smooth surface without any substance. Figure [45]. On the other hand, this may be related to the high specific surface area of CFs [46]. Figure 2e shows the sandwich structure CF@(CNT/Fe 3 O 4 )/EP composite obtained after rolling, with CFs in the middle and the thickness of (CNT/Fe 3 O 4 )/EP distributed on both sides is about 2 µm, which is uniformly attached to the CFs surface. It can be clearly seen in Figure 2f As is shown in Figure 2a, the left side is pure CFs, and the middle and right side are the CFs sprayed with CNT/Fe3O4 dispersion fog. It can be clearly seen that pure CFs has a light color and luster, but the CFs sprayed with CNT/Fe3O4 dispersion fog appears darker color. This is because the adsorption of CNTs and Fe3O4 on the CFs surface and changes the diffuse reflection of CFs surface. Figure 2b shows that pure CFs has a smooth surface without any substance. Figure 2c,d show the attachment of CNT/Fe3O4 when CFs moving speed is 0.01 m/s and 0.02 m/s, respectively. The faster CFs moving speed, the less CNT/Fe3O4 is deposited, and the lighter color is appeared on the macroscopic (Figure 2a, right). At the same time, CNTs and Fe3O4 have high dispersion degree and without obvious agglomeration, CNTs is connected to each other and extend to the radial and axial directions of CFs, presenting a 3D distribution, Fe3O4 is interspersed in the CNTs network, and adsorbed on the CFs surface. On the one hand, CNTs and Fe3O4 are coated by the size agent on the CFs surface, which establishes the physical association between CFs and CNT/Fe3O4 [45]. On the other hand, this may be related to the high specific surface area of CFs [46]. Figure 2e shows the sandwich structure CF@(CNT/Fe3O4)/EP composite obtained after rolling, with CFs in the middle and the thickness of (CNT/Fe3O4)/EP distributed on both sides is about 2 μm, which is uniformly attached to the CFs surface. It can be clearly seen in Figure 2f,g that CNTs and Fe3O4 are coated in EP, in which CNTs is attached to the CFs surface and distribute in the axial direction of CFs only, and Fe3O4 is interspersed in CNTs network with uniform distribution. This morphology is obviously different from Figure 2c,d; this indicates that the effect of rolling makes CNT/Fe3O4 change from 3D to 2D planar structure, which is conducive to maintaining the insulation between CFs layers. Considering that CNTs and Fe3O4 may change their properties under the action of free arc, Fe3O4 may be converted into Fe2O3 at high temperature [47]. The Raman of CNT/Fe3O4 dispersion fog obtained using the free arc dispersion method is compared with pure CNTs (Figure 3a). CNT/Fe3O4 dispersion fog has characteristic peaks at 1341 cm −1 (Dline) and 1578 cm −1 (G-line), which are the characteristic peaks of carbonaceous com- Considering that CNTs and Fe 3 O 4 may change their properties under the action of free arc, Fe 3 O 4 may be converted into Fe 2 O 3 at high temperature [47]. The Raman of CNT/Fe 3 O 4 dispersion fog obtained using the free arc dispersion method is compared with pure CNTs (Figure 3a). CNT/Fe 3 O 4 dispersion fog has characteristic peaks at 1341 cm −1 (D-line) and 1578 cm −1 (G-line), which are the characteristic peaks of carbonaceous compounds [48,49]; this parameter complies with the CNT standard spectrum. In addition, the I D /I G values of CNTs/Fe 3 O 4 and CNTs are 1.15 and 1.12, respectively; this indicates that the graphitization degree of CNTs is not affected by the free arc. Figure 3b shows the comparison of CNT/Fe 3 O 4 dispersion fog Raman image and Fe 3 O 4 standard spectrum, and the result is also consistent [50,51]. It demonstrated that the structure of CNTs and  Radial thermal conductivity (λ⊥) of the multilayer CF@(CNT/Fe3O4)/EP-CFs composite is shown in Figure 4a. The λ⊥ of CF-0 is 0.38 W/(m·K), and the λ⊥ of CF-2 to CF-7 remains stable at about 0.38 W/(m·K) with the increase of CNT/Fe3O4. This is because CNT/Fe3O4 changes from 3D to 2D plane due to the rolling treatment. CNTs with excellent thermal conductivity (about 3000 W/(m·K)) [54] are attached to the CF surface and covered by EP [8] with high insulation, forming (CNT/Fe3O4)/EP. It makes cross-plane heat conduction in the multilayer CF@(CNT/Fe3O4)/EP-CFs composite not easy. In addition, due to the presence of contact thermal resistance between CFs [55], the CFs core sheath in CFs 2 to CFs 7 forms radial thermal insulation layer. Both (CNT/Fe3O4)/EP and CFs core sheath form the multilayer thermal insulation system. λ⊥ of CF-8 increases slightly. From CF-7 to CF-8, λ⊥ increases from 0.38 W/(m·K) to 0.44 W/(m·K). The main reason is that CF-8 does not contain a CFs core sheath; therefore, the radial thermal insulation layer is lost, but because of the (CNT/Fe3O4)/EP, the increase of λ⊥ is not significant. Radial thermal conductivity (λ ⊥ ) of the multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite is shown in Figure 4a. The λ ⊥ of CF-0 is 0.38 W/(m·K), and the λ ⊥ of CF-2 to CF-7 remains stable at about 0.38 W/(m·K) with the increase of CNT/Fe 3 O 4 . This is because CNT/Fe 3 O 4 changes from 3D to 2D plane due to the rolling treatment. CNTs with excellent thermal conductivity (about 3000 W/(m·K)) [54] are attached to the CF surface and covered by EP [8] with high insulation, forming (CNT/Fe 3 O 4 )/EP. It makes cross-plane heat conduction in the multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite not easy. In addition, due to the presence of contact thermal resistance between CFs [55], the CFs core sheath in CFs 2 to CFs 7 forms radial thermal insulation layer. Both (CNT/Fe 3 O 4 )/EP and CFs core sheath form the multilayer thermal insulation system. λ ⊥ of CF-8 increases slightly. From CF-7 to CF-8, λ ⊥ increases from 0.38 W/(m·K) to 0.44 W/(m·K). The main reason is that CF-8 does not contain a CFs core sheath; therefore, the radial thermal insulation layer is lost, but because of the (CNT/Fe 3 O 4 )/EP, the increase of λ ⊥ is not significant.

Results and Discussion
Axial thermal conductivity (λ ) of the multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite is shown in Figure 4b. Different from λ ⊥ , with the increase of CNT/Fe 3 O 4 content, λ increases. When the amount of CNT/Fe 3 O 4 is from 0 to 0.56 mg/cm 3 (CF-0 to CF-7), λ increases from 0.59 W/(m·K) to 1.1 W/(m·K), and the growth rate is 86%. The reason is that CFs has excellent thermal conductivity [56], and the axial heat conduction is not affected by EP and CFs core sheath. In addition, CNTs in the (CNT/Fe 3 O 4 )/EP forms a good thermal conductivity network [57]; the higher CNTs content, the more abundant CNTs network, and the higher heat conduction efficiency. When the amount of CNT/Fe 3 O 4 is changed from 0.56 to 0.64 mg/cm 3 (CF-7 to CF-8), λ hardly changes, but the instability (standard deviation) increases. The main reason is that CF-8 does not have CFs core sheath, the barrier of radial heat conduction is greatly reduced, so the heat conduction has a component in radial. Thus, the increase in axial thermal conductivity is limited and the heat transfer randomness is increased. Axial thermal conductivity (λ‖) of the multilayer CF@(CNT/Fe3O4)/EP-CFs composite is shown in Figure 4b. Different from λ⊥, with the increase of CNT/Fe3O4 content, λ‖ increases. When the amount of CNT/Fe3O4 is from 0 to 0.56 mg/cm 3 (CF-0 to CF-7), λ‖ increases from 0.59 W/(m·K) to 1.1 W/(m·K), and the growth rate is 86%. The reason is that CFs has excellent thermal conductivity [56], and the axial heat conduction is not affected by EP and CFs core sheath. In addition, CNTs in the (CNT/Fe3O4)/EP forms a good thermal conductivity network [57]; the higher CNTs content, the more abundant CNTs network, and the higher heat conduction efficiency. When the amount of CNT/Fe3O4 is changed from 0.56 to 0.64 mg/cm 3 (CF-7 to CF-8), λ‖ hardly changes, but the instability (standard deviation) increases. The main reason is that CF-8 does not have CFs core sheath, the barrier of radial heat conduction is greatly reduced, so the heat conduction has a component in radial. Thus, the increase in axial thermal conductivity is limited and the heat transfer randomness is increased.
The difference between λ‖ and λ⊥ indicates that the CFs core sheath and (CNT/Fe3O4)/EP have influence on the thermal anisotropy of multilayer CF@(CNT/Fe3O4)/EP-CFs composite. Figure 4c directly represents the difference between λ‖ and λ⊥of multilayer CF@(CNT/Fe3O4)/EP-CFs composite, the larger value of λ‖/λ⊥, the more significant thermal anisotropy. From CF-0 to CF-7, λ‖/λ⊥ gradually increases, while CF-7 to CF-8 starts to decrease. Obviously, the λ‖/λ⊥ of CF-7 is higher than CF-8; this is attributed to the CFs core sheath in CF-7. On the one hand, the CFs core sheath stabilizes λ⊥; on the other hand, the CFs core sheath eliminates the radial component of heat conduction to ensure λ‖ promotion. For CF-8, the large λ⊥ and the similar λ‖ make its thermal anisotropy insignificant compared to CF-7. Figure 4d shows that within the 30-200 °C, the The difference between λ and λ ⊥ indicates that the CFs core sheath and (CNT/Fe 3 O 4 )/ EP have influence on the thermal anisotropy of multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite. Figure 4c directly represents the difference between λ and λ ⊥ of multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite, the larger value of λ /λ ⊥ , the more significant thermal anisotropy. From CF-0 to CF-7, λ /λ ⊥ gradually increases, while CF-7 to CF-8 starts to decrease. Obviously, the λ /λ ⊥ of CF-7 is higher than CF-8; this is attributed to the CFs core sheath in CF-7. On the one hand, the CFs core sheath stabilizes λ ⊥ ; on the other hand, the CFs core sheath eliminates the radial component of heat conduction to ensure λ promotion. For CF-8, the large λ ⊥ and the similar λ make its thermal anisotropy insignificant compared to CF-7. Figure 4d shows that within the 30-200 • C, the λ ⊥ of CF-8 with rolling treatment (about 0.4 W/(m·K)) is lower than no rolling treatment (about 1.4 W/(m·K)). This shows from the performance point of view that the (CNT/Fe 3 O 4 )/EP formed by rolling can effectively reduce the heat transfer between layers. Combined with the difference between λ and λ ⊥ , the main reason is that CNTs have high axial thermal conductivity [58], and rolling makes CNTs attach to the CFs surface and extend along the axial direction of CFs. At this time, heat can be transferred along the CFs axial direction; however, due to the coverage of EP and the direction of CNTs, radial heat transfer is difficult. Figure 5a. The σ ⊥ of CF-0 to CF-7 is generally stable, maintaining at 1.1 × 10 −4 (S/cm), while the σ ⊥ of CF-8 is increased to 1.7 × 10 −4 (S/cm), showing a slight improvement. The main reason is that CNTs has low resistance/high electrical conductivity (10 5 -10 7 S/m) [59,60], and adding CNTs to the composite can improve electrical conductivity. Similar to the thermal conductivity, due to the insulation effect of (CNT/Fe 3 O 4 )/EP and CFs core sheath, the cross-plane electrical conduction in multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite is difficult to carry out. Because CF-8 does not have CFs core sheath and CNTs enhance the electrical conductivity of EP [61], so the σ ⊥ of CF-8 obtains some improvement.

Radial electrical conductivity (σ ⊥ ) of multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite is shown in
(about 1.4 W/(m·K)). This shows from the performance point of view that the (CNT/Fe3O4)/EP formed by rolling can effectively reduce the heat transfer between layers. Combined with the difference between λ‖ and λ⊥, the main reason is that CNTs have high axial thermal conductivity [58], and rolling makes CNTs attach to the CFs surface and extend along the axial direction of CFs. At this time, heat can be transferred along the CFs axial direction; however, due to the coverage of EP and the direction of CNTs, radial heat transfer is difficult.
Radial electrical conductivity (σ⊥) of multilayer CF@(CNT/Fe3O4)/EP-CFs composite is shown in Figure 5a. The σ⊥ of CF-0 to CF-7 is generally stable, maintaining at 1.1 × 10 −4 (S/cm), while the σ⊥ of CF-8 is increased to 1.7 × 10 −4 (S/cm), showing a slight improvement. The main reason is that CNTs has low resistance/high electrical conductivity (10 5 -10 7 S/m) [59,60], and adding CNTs to the composite can improve electrical conductivity. Similar to the thermal conductivity, due to the insulation effect of (CNT/Fe3O4)/EP and CFs core sheath, the cross-plane electrical conduction in multilayer CF@(CNT/Fe3O4)/EP-CFs composite is difficult to carry out. Because CF-8 does not have CFs core sheath and CNTs enhance the electrical conductivity of EP [61], so the σ⊥ of CF-8 obtains some improvement. As shown in Figure 5b, axial electrical conductivity (σ‖) is higher than σ⊥, the σ‖ of CF-0, CF-8 and CF@CNTs (the content of CNTs is 0.64 mg/cm 3 ) are 6.2 S/m, 7.7 S/m and 9.4 S/m, respectively, showing increase trend. The main reason is that σ‖ is not restricted by (CNT/Fe3O4)/EP, CFs core sheath and interlamination contact resistance, and CFs have high axial electrical conductivity (about 670 S/cm) [62]. The σ‖ of CF-8 is higher than CF-0 because CNTs is contained in the filler, and the CNTs direction is along the CFs axial direction, which helps to improve the axial electrical conductivity of EP and composite. In addition, although the same mass of CNTs (0.64 mg/cm 3 ) is added in CF@CNT composite, the σ‖ of CF@CNT is higher than CF-8. This is attributed to the fact that Fe3O4 has poor electrical conductivity [63], CF-8 contains 0.48 mg/cm 3 Fe3O4 and CNTs content is much lower than CF@CNT, which makes low electrical conductivity for CF-8. Figure 5c shows that the interlaminar resistivity of multilayer CF@(CNT/Fe3O4)/EP-CFs composite with rolling is generally higher than no rolling. The main reason is that CNTs no rolling may penetrate EP, thus connecting adjacent CFs, forming CFs-CNTs-CFs interlayer electric conduction pathway, which reduces the macroscopic resistivity and influences the interlamination insulation performance of the composite.
The magnetic property of CFs, CNT/Fe3O4 powder and CF@(CNT/Fe3O4)/EP are tested, and the results are shown in Figure 6a. CFs has no magnetic, and the saturation magnetization (Ms) of CNT/Fe3O4 powder is 40 emu/g, when combined with CFs and EP, the Ms decreases to 2.6 emu/g, which is mainly attributed to CNT/Fe3O4 is coated [64]. It As shown in Figure 5b, axial electrical conductivity (σ ) is higher than σ ⊥ , the σ of CF-0, CF-8 and CF@CNTs (the content of CNTs is 0.64 mg/cm 3 ) are 6.2 S/m, 7.7 S/m and 9.4 S/m, respectively, showing increase trend. The main reason is that σ is not restricted by (CNT/Fe 3 O 4 )/EP, CFs core sheath and interlamination contact resistance, and CFs have high axial electrical conductivity (about 670 S/cm) [62]. The σ of CF-8 is higher than CF-0 because CNTs is contained in the filler, and the CNTs direction is along the CFs axial direction, which helps to improve the axial electrical conductivity of EP and composite. In addition, although the same mass of CNTs (0.64 mg/cm 3 ) is added in CF@CNT composite, the σ of CF@CNT is higher than CF-8. This is attributed to the fact that Fe 3 O 4 has poor electrical conductivity [63], CF-8 contains 0.48 mg/cm 3 Fe 3 O 4 and CNTs content is much lower than CF@CNT, which makes low electrical conductivity for CF-8. Figure 5c shows that the interlaminar resistivity of multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite with rolling is generally higher than no rolling. The main reason is that CNTs no rolling may penetrate EP, thus connecting adjacent CFs, forming CFs-CNTs-CFs interlayer electric conduction pathway, which reduces the macroscopic resistivity and influences the interlamination insulation performance of the composite.
The magnetic property of CFs, CNT/Fe 3 O 4 powder and CF@(CNT/Fe 3 O 4 )/EP are tested, and the results are shown in Figure 6a. CFs has no magnetic, and the saturation magnetization (Ms) of CNT/Fe 3 O 4 powder is 40 emu/g, when combined with CFs and EP, the Ms decreases to 2.6 emu/g, which is mainly attributed to CNT/Fe 3 O 4 is coated [64]. It can be seen from local magnification (Figure 6b can be seen from local magnification (Figure 6b) that the coercivity (Hc) of CNT/Fe3O4 powder and CF@(CNT/Fe3O4)/EP are 66.6Oe and 64.6Oe, respectively. The similar Hc values indicate that free arc has no effect on the antidemagnetization ability of CNT/Fe3O4. The SET, SER and SEA of CF-0 to CF-8 are shown in Figure 6c-e. The higher CNT/Fe3O4 content, the higher SET, SER and SEA value, and the SEA value is greater than SER. In CF@(CNT/Fe3O4)/EP, electromagnetic wave interacts with Fe3O4 first when passing through (CNT/Fe3O4)/EP, part of the electromagnetic wave is absorbed due to hysteresis loss and natural resonance, and the rest will reach the CFs surface. Here, a part of electromagnetic wave is reflected back to (CNT/Fe3O4)/EP due to impedance mismatch, and the remaining part will pass through CFs to (CNT/Fe3O4)/EP on the other side [55]. CF@(CNT/Fe3O4)/EP with sandwich structure attenuates electromagnetic wave by multiple absorption, reflection and scattering processes and improves its EMI shielding performance [65][66][67]. Due to multilayer CF@(CNT/Fe3O4)/EP-CFs composite having more than one layer of CF@(CNT/Fe3O4)/EP, it provides more opportunities for electromagnetic wave propagation, so the above attenuation process of absorption-reflection/penetrationreabsorption for electromagnetic wave will be repeated many times and strengthens EMI shielding performance. In this process, since absorption is the main attenuation mode of electromagnetic wave, so the value of SEA is greater than SER. In addition, as the electromagnetic wave is absorbed, the heat (converted by the electromagnetic wave) generated by the electrical loss and magnetic loss accumulates inside the composite, which will cause the composite temperature increase. Figure 6f shows the average values (SEave) of SET, SER and SEA in X-band (8.2-12.4 GHz). When the amount of CNT/Fe3O4 is 0 (CF-0), the SEave of SET, SER and SEA are 10.56 dB, 2.03 dB and 8.53 dB, respectively. When the addition of CNT/Fe3O4 is increased to 0.64 mg/cm 3 (CF-8), compared with CF-0, the SEave of SET, SER and SEA are increased by 172%, 159% and 175% respectively. The reason is that CF-8 contains Fe3O4 while CF-0 does not, so the magnetic loss and dielectric loss for Fe3O4 are missing, which greatly reduces the The SE T , SE R and SE A of CF-0 to CF-8 are shown in Figure 6c-e. The higher CNT/Fe 3 O 4 content, the higher SE T , SE R and SE A value, and the SE A value is greater than SE R . In CF@(CNT/Fe 3 O 4 )/EP, electromagnetic wave interacts with Fe 3 O 4 first when passing through (CNT/Fe 3 O 4 )/EP, part of the electromagnetic wave is absorbed due to hysteresis loss and natural resonance, and the rest will reach the CFs surface. Here, a part of electromagnetic wave is reflected back to (CNT/Fe 3 O 4 )/EP due to impedance mismatch, and the remaining part will pass through CFs to (CNT/Fe 3 O 4 )/EP on the other side [55]. CF@(CNT/Fe 3 O 4 )/EP with sandwich structure attenuates electromagnetic wave by multiple absorption, reflection and scattering processes and improves its EMI shielding performance [65][66][67]. Due to multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite having more than one layer of CF@(CNT/Fe 3 O 4 )/EP, it provides more opportunities for electromagnetic wave propagation, so the above attenuation process of absorption-reflection/penetrationreabsorption for electromagnetic wave will be repeated many times and strengthens EMI shielding performance. In this process, since absorption is the main attenuation mode of electromagnetic wave, so the value of SE A is greater than SE R . In addition, as the electromagnetic wave is absorbed, the heat (converted by the electromagnetic wave) generated by the electrical loss and magnetic loss accumulates inside the composite, which will cause the composite temperature increase. Figure 6f shows the average values (SE ave ) of SE T , SE R and SE A in X-band (8.2-12.4 GHz). When the amount of CNT/Fe 3 O 4 is 0 (CF-0), the SE ave of SE T , SE R and SE A are 10.56 dB, 2.03 dB and 8.53 dB, respectively. When the addition of CNT/Fe 3 O 4 is increased to 0.64 mg/cm 3 (CF-8), compared with CF-0, the SE ave of SE T , SE R and SE A are increased by 172%, 159% and 175% respectively. The reason is that CF-8 contains Fe 3 O 4 while CF-0 does not, so the magnetic loss and dielectric loss for Fe 3 O 4 are missing, which greatly reduces the electromagnetic wave absorption effect and leads to relatively poor EMI shielding performance [68,69]. Table 2 summarizes the λ, σ and EMI SE for some related polymer composites; it is observed that multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite prepared by this work has good performances.

Conclusions
In this work, the multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite is obtained by free arc dispersion-CFs surface spraying-rolling process method and structural design. Under circumstance of the content for CNT/Fe 3 O 4 is very small, the (CNT/Fe 3 O 4 )/EP and CFs core sheath achieve thermal and electrical anisotropy and directional enhancement for multilayer CF@(CNT/Fe 3 O 4 )/EP-CFs composite, multilayer sandwich structure makes the EMI shielding performance better strengthened by multiple absorption-reflection/penetrationreabsorption of electromagnetic wave. From CF-0 to CF-8, the content of CNT/Fe 3 O 4 only increases by 0.045 wt%, λ increases from 0.59 W/(m·K) to 1.1 W/(m·K), the growth rate is 86%, λ ⊥ only increases by 0.05 W/(m·K), and the maximum λ /λ ⊥ is 2.9, σ increases from 6.2 S/cm to 7.7 S/cm, growth rate is 24%, σ ⊥ only increases by 0.7 × 10 −4 S/cm and EMI SE T increases by 196%, from 10.3 dB to 30.5 dB. This provides a new idea for enhancing CFs composite properties.